Substrate for magnetic recording media, manufacturing method for the same, and magnetic recording media

ABSTRACT

A substrate surface polishing step in the manufacturing method for a magnetic recording medium glass substrate uses a polishing solution consisting of a mixture solution of water and abrasive grains with potassium hydroxide. One embodiment of the invention uses only a mixture solution of water and potassium hydroxide to manufacture a glass substrate with very fine surface roughness. A second embodiment uses a two-step process in which the glass substrate is first polished using water, abrasive grains and potassium hydroxide, and is then again polished using water and potassium hydroxide, but omitting abrasive grains. A glass substrate, produced according to the invention, results in the manufacture of a magnetic recording medium with excellent high recording density.

BACKGROUND TO THE INVENTION

[0001] The present invention relates to a glass substrate as anon-magnetic substrate for a magnetic recording medium mounted ontovarious magnetic recording devices such as an external memory device forcomputers and the like. The present invention also relates to amanufacturing method for the same, and to a magnetic recording medium.

[0002] For the substrate used in thin film magnetic recording media(henceforth referred to as media), in order to achieve high recordingdensity, surface shape accuracy and mechanical properties must bemaintained through a high degree of precision processing and precisionpolishing. With increased recording volume of the magnetic recordingdevice, low-floating magnetic heads are needed, and the necessity forhigh precision is even greater.

[0003] In recent years, instead of the aluminum alloy substrate that islargely used in the prior art, glass substrates that are stronger andhave little surface shaking during high speed rotation is becoming moremainstream as the medium substrate.

[0004] For manufacturing glass substrates, a polish processing isgenerally conducted for polishing the surface. In general, polishprocessing is a method for polishing the substrate surface, wherein twoplatens are placed one on top of the other as a unit; pads are attachedto the surfaces where the lower and upper platens contact each other;polishing solution such as slurry and the like is poured between theupper and lower pads while rotating the platens; a substrate is anchoredby a carrier between the upper and lower pads, and the substrate surfaceis polished. This is a total surface contact processing where thepolishing is conducted by having the entire surface of the substratecontacted at the same time by the pads that are the processing members.On top of the glass substrate that has been surface polished, a Cr underlayer, Co alloy magnetic layer, and a carbon protective film and thelike are formed by a sputtering method. On top of this, a liquidlubricant layer is coated, thereby forming the medium.

[0005] The glass substrate used for the medium, polished as describedabove, has a very fine surface roughness. A polishing solution, forexample a mixture solution of polishing abrasive grains of cerium oxideabrasive grains and water, is used for polish processing. However,adherence of abrasive grains to the substrate surface and smallscratches by the abrasive grains cannot be avoided.

[0006] In recent years, the need for higher recording volume formagnetic recording devices is even greater, and there is demand forincreasing the recording density of media. As a result, magnetic headshave become low-floating, and there is demand for further reduction insurface roughness of the substrate used by media.

[0007] By decreasing the amount of abrasive grains in the polishingsolution used for polish processing the glass substrate, the amount ofadhesion of the abrasive grains to the substrate surface is reduced, andthe generation of small scratches by the abrasive grains is reduced. Forexample, when using cerium oxide grains as the polishing abrasivegrains, the concentration of the abrasive grains in the prior art hasbeen generally 10 weight % to 20 weight %. By reducing thisconcentration, the amount of adhesion of cerium oxide to the substratesurface is reduced, and the small scratches by the abrasive grains isreduced. However, this is not a fundamental solution, and there is alsothe disadvantage of a reduced amount of processing.

OBJECTS AND SUMMARY OF THE INVENTION

[0008] Upon considering the above points, the object of the presentinvention is to provide a glass substrate with very low surfaceroughness and a manufacturing method for the same. A further object isto provide a medium that uses this substrate to attain an excellent highrecording density.

[0009] The present invention is used for manufacture of a glasssubstrate used as a non-magnetic substrate for a magnetic recordingmedium that comprises a sequential layering of at least a non-magneticunder layer, a magnetic layer, a protective layer, and a liquidlubricant layer on top of the non-magnetic substrate. The aboveobjective is solved by using a mixture solution of water, polishingabrasive grains, and potassium hydroxide for the polishing solution ofthe surface polishing step in the manufacturing method for the aboveglass substrate.

[0010] By having the polishing solution contain potassium hydroxide, asubstrate with very fine surface roughness is obtained without reducingthe amount of processing. This is due to the potassium hydroxide whichhas an etching effect and also has the effect of reducing frictionduring processing.

[0011] The potassium hydroxide concentration contained in the polishingsolution is preferably in the range of 0.1 weight % or greater and 15weight % or less. If the concentration is less than 0.1 weight %, therequired etching effect is barely achieved, and if the concentrationexceeds 15 weight %, the etching becomes too strong, and negativeeffects such as pitting results.

[0012] Furthermore, as the abrasive grains mixed in the polishingsolution, cerium oxide abrasive grains generate few scratches and aresuitable.

[0013] As the method for surface polishing, the polish processingmethod, which is a total surface contact processing method whereinpolishing is conducted by simultaneous contacting of the entiresubstrate surface to the processing member, is used.

[0014] Furthermore, for the surface polishing method, if a partialcontact processing method, wherein polishing is conducted by having onesection of a processing member that moves in one direction contact onesection of a rotating substrate surface, clogging or deterioration ofthe pad is eliminated, and a very fine surface roughness is easilyobtained. The contact surface area where the processing member contactsthe substrate surface is preferably 2% or greater and 30% or less of theentire substrate surface area. If it is less than 2%, too much time isneeded for processing, and if it exceeds 30%, a very fine surfaceroughness is harder to obtain.

[0015] Furthermore, for the polishing solution, if abrasive grains suchas cerium oxide and the like are not added and a polishing solution inwhich only water and potassium hydroxide are added is used, a surfaceroughness of a center line average roughness Ra of 0.2 nm or less isobtained. This is a very fine surface roughness that is not seen in theprior art.

[0016] If the glass substrate is a chemically strengthened glasssubstrate, the above polishing method is suitable for the surfacepolishing prior to chemical strengthening as well as for polishing inorder to remove surface irregularities generated by chemical treatmentafter the chemical strengthening treatment.

[0017] A substrate for a magnetic recording medium is created with themanufacturing method containing one of the polishing steps as describedabove. By creating a magnetic recording medium using this substrate, amedium with excellent high recording density is obtained.

[0018] As the surface polishing step in the manufacturing method for theglass substrate, a standard lapping of the glass substrate surface isconducted.

[0019] Next, after lapping, in order to have an even finer surfaceroughness, fine surface polishing is conducted on the glass substratesurface.

[0020] For the fine surface polishing, polish processing, which is aprocessing method wherein there is polishing by simultaneous contact ofthe entire surface of the substrate surface by the processing memberthat conducts polishing, is used. In polish processing, a device havinga construction of the standard polish processing device is suitable. Inthe standard polish processing device, two platens are placed one on topof the other as a unit; pads are attached to the surfaces where thelower and upper platens contact each other; polishing solution issupplied between the upper and lower pads while rotating the platens; asubstrate is anchored by a carrier between the upper and lower pads, andthe substrate surface is polished.

[0021] Or else, instead of polish processing, a partial contactprocessing, wherein polishing is conducted by contacting one section ofa rotating substrate surface with one section of a processing memberthat moves in one direction, is used. The surface area of the contact ofthe processing member to the substrate surface is 2% or greater and lessthan 30% of the substrate surface area.

[0022] In order to conduct partial contact processing, for example, apolishing device shown principally in FIG. 1 is suitable. FIG. 1(a) is afront view, and FIG. 1(b) is a side view. Referring to FIG. 1, asubstrate 1 rotates in the direction of arrow A. Polishing members 2 areeach pressed into contact with the two sides of substrate 1 by forcingjigs 3. While each of polishing members 2 is moving in the direction ofarrow B, polishing solution is supplied to the contact surface bynozzles 4, and the substrate surface is polished. Polishing members 2have a width sufficient for covering the width that is to be processedin the radial direction of substrate 1. The shape of the surface offorcing jigs 3 contacting the substrate surface and the pushing force offorcing jigs 3 is selected as appropriate depending on the surfaceroughness that is to be achieved for the substrate.

[0023] For the polishing solution, a mixture solution in which potassiumhydroxide is added to a mixture solution of water and abrasive grains toobtain a potassium hydroxide concentration of 0.1 weight % to 15 weight% is used. By using this polishing solution, a substrate with a veryfine surface roughness is achieved. For the abrasive grains, any of thestandard types of grains is used, but in order to reduce scratching,cerium oxide is preferably used.

[0024] Furthermore, in the surface polishing step described above, byusing a polishing solution of a mixture solution that has only water andpotassium hydroxide and does not contain abrasive grains, a very finesurface roughness is achieved.

[0025] By using a glass substrate manufactured in this manner, a mediumwith excellent high recording density is achieved.

[0026] The above, and other objects, features and advantages of thepresent invention will become apparent from the following descriptionread in conjunction with the accompanying drawings, in which likereference numerals designate the same elements.

BRIEF DESCRIPTION OF THE DRAWINGS

[0027] FIGS. 1(a) and 1(b) are explanatory drawings of a model of oneexample of a polishing device and polishing method using the partialcontact processing relating to the present invention.

[0028]FIG. 2 is a surface image by AFM (atomic force microscope) of thesubstrate surface obtained by Embodiment 2.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0029] Concrete embodiments of the present invention is described below.

[0030] Embodiment 1

[0031] Lapping was conducted on a circular glass substrate with adiameter of 2.5 inches. After lapping, polish processing of thesubstrate was conducted. A foam urethane pad was used as the polishingmember. For the polishing solution, a mixture solution was used, inwhich 1 weight % of cerium oxide abrasive grains with an average grainsize of 1.5 micrometers and 5 weight % of potassium hydroxide was addedto water. The surface roughness Ra prior to polish processing was 10 nmor greater. After processing, the surface roughness was finer at 0.4 nm.

[0032] Embodiment 2

[0033] With the polish processed substrate obtained in Embodiment 1,polishing was conducted using the polishing device shown in FIG. 1. Apolishing tape of a foam urethane pad was used as the polishing member.The contact surface area of the polishing member with respect to thesubstrate surface area was 20%. The pressure applied to the surface ofthe substrate was 40 g/cm². The rotation speed of the substrate was 200rpm. The polishing solution omitted cerium oxide abrasive grains.

[0034] After processing, the surface roughness of the substrate wasexamined with an Atomic Force Microscope (AFM). Referring to FIG. 2, thesurface image is shown. The surface Ra prior to processing was 0.4 nm.In contrast, after processing, a very fine surface roughness of 0.2 nmwas achieved.

[0035] According to the present invention, in the surface polishing stepof the manufacturing method for the glass substrate used as thenon-magnetic substrate of a magnetic recording medium, a polishingsolution containing potassium hydroxide is used. As a result, asubstrate with a finer surface roughness is manufactured. Furthermore,by using a mixture solution that does not contain abrasive grains andcontains only water and potassium hydroxide as the polishing solution, asubstrate with a very fine surface roughness is manufactured. By usingthis substrate, a medium with excellent high recording density isobtained.

[0036] There exist circumstances in which the polishing using onlyembodiment 2 is satisfactory. That is, polishing with an alkali solutionbut without the use of cerium oxide grains. This is effective forremoving slight surface projections on the chemically strengthenedsurface. When only embodiment 1 is used, a relatively rough surface isattained, and at least some cerium oxide grains remain on the surfaceafter polishing is completed. Thus, for a smoother surface (about 1 nm)the polishing of embodiment 2 may follow the polishing of embodiment 1.When this is done, residual cerium oxide grains are removed, and thesurface smoothness is improved.

[0037] Having described preferred embodiments of the invention withreference to the accompanying drawings, it is to be understood that theinvention is not limited to those precise embodiments, and that variouschanges and modifications may be effected therein by one skilled in theart without departing from the scope or spirit of the invention asdefined in the appended claims.

What is claimed is:
 1. A manufacturing method for a glass substrate fora magnetic recording medium, comprising: mixing a polishing solution ofwater and abrasive grains with potassium hydroxide; and polishing asurface of said glass substrate with said polishing solution.
 2. Amanufacturing method according to claim 1 , wherein: a concentration ofpotassium hydroxide in said polishing solution is from about 0.1 weight% to about 15 weight %.
 3. A manufacturing method according to claim 1 ,wherein said abrasive grains are cerium oxide.
 4. A manufacturing methodaccording to claim 2 , wherein said abrasive grains are cerium oxide. 5.A manufacturing method for a substrate for a magnetic recording medium:mixing a polishing solution containing at least water and potassiumhydroxide; polishing a surface of said magnetic recording medium by atotal surface contact processing method wherein polishing is conductedby simultaneous contact of a processing member with an entire surface ofsaid substrate surface.
 6. A manufacturing method for a substrate for amagnetic recording medium, comprising: mixing a polishing solutioncontaining at least water and potassium hydroxide; surface polishing asurface of said substrate using said polishing solution; said surfacepolishing is by a partial contact processing method wherein polishing isconducted by having a first section of a rotating substrate surfacecontact a second section of a processing member that is moving.
 7. Amanufacturing method for a substrate for a magnetic recording medium,according to claim 6 , wherein: during surface polishing, a contactsurface area of said first section is from about 2% to about 30% of atotal surface area of said substrate.
 8. A manufacturing method for asubstrate for a magnetic recording medium, comprising: mixing apolishing solution containing at least water and potassium hydroxide;surface polishing a surface of said substrate using said polishingsolution; and then chemically strengthening said glass substrate.
 9. Asubstrate for a magnetic recording medium having a surface polishedusing a mixture of water and at least potassium hydroxide.
 10. Amagnetic recording medium, comprising: a magnetic recording mediumsubstrate having a polished surface polished using a mixture of waterand at least potassium hydroxide; sequential layering of at least amagnetic layer on said surface; layering at least a protective layer onsaid magnetic layer.
 11. A manufacturing method for a glass substratefor a magnetic recording medium, comprising: mixing a polishing solutionof water and potassium hydroxide; and polishing a surface of said glasssubstrate with said polishing solution.
 12. A manufacturing method for aglass substrate for a magnetic recording medium, comprising: mixing afirst polishing solution of water, potassium hydroxide and an abrasive;polishing a surface of said glass substrate with said first polishingsolution; mixing a second polishing solution of water and potassiumhydroxide; and polishing said surface with said second polishingsolution until a residue of said abrasive is substantially removed, anda desired surface smoothness is attained.